The present invention relates generally to implantable pacemakers and more particularly to a subcutaneous multiple electrode sensing and recording system for acquiring electrocardiographic data and waveform tracings from an implanted pacemaker without the need for or use of surface (skin) electrodes. More particularly, the present invention relates to implantable devices that are equipped with a compliant, insulative xe2x80x9cshroudxe2x80x9d into which are placed electrodes (a Subcutaneous Electrode Array or SEA) that, in turn, detect cardiac depolarizations communicable and displayable by a portable device programmer.
The electrocardiogram (ECG) is commonly used in medicine to determine the status of the electrical conduction system of the human heart. As practiced the ECG recording device is commonly attached to the patient via ECG leads connected to pads arrayed on the patient""s body so as to achieve a recording that displays the cardiac waveforms in any one of 12 possible vectors.
The history of the ECG dates back to 1842 when the Italian physicist, Carlo Matteucci discovered that each heartbeat was accompanied by a detectable electric signal (Matteucci C. Sur un phenomene physiologique produit par les muscles en contraction. Ann Chim Phys 1842;6:339-341). In 1878, two British physiologists, John Burden Sanderson and Frederick Page, determined that the heart signal consisted of, at least, two phases, the QRS (ventricular depolarization) and the repolarization or T-wave (Burdon Sanderson J. Experimental results relating to the rhythmical and excitatory motions of the ventricle of the frog. Proc R Soc Lond 1878;27:410-414). It was not until 1893, however, that Willem Einthoven introduced the term xe2x80x98electrocardiogramxe2x80x99 at a meeting of the Dutch Medical Association (Einthoven W: Nieuwe methoden voor clinisch onderzoek [New methods for clinical investigation]. Ned T Geneesk 29 II: 263-286, 1893), although he later disavowed he was the originator of the term. Einthoven may, however, be called the Father or electrocardiography, since he won the Nobel Prize for his achievements in 1924. It was he who finally dissected and named all of the cardiac waveforms (P, Q, R, S, T) that commonly appear on an ECG tracing from a xe2x80x98normalxe2x80x99 person (Einthoven W. Ueber die Form des menschlichen Electrocardiogramms. Archf d Ges Physiol 1895;60:101-123).
Einthoven and other medical practitioners of that time were aware of only three vectors (I, II, and III) that are achieved by placement of the ECG electrodes on specific body sites. The remaining nine sites were discovered later in the twentieth century. In 1938, American Heart Association and the Cardiac Society of Great Britain defined the standard positions (I-III) and wiring of the chest leads V1-V6. The xe2x80x98Vxe2x80x99 stands for voltage. (Barnes AR, Pardee HEB, White PD. et al. Standardization of precordial leads. Am Heart J 1938;15:235-239). Finally, in 1942, Emanuel Goldberger added the augmented limb leads aVR, aVL and aVF to Einthoven""s three limb leads and the six chest leads thereby creating the 12-lead electrocardiogram that is routinely used today for cardiac diagnostic purposes. A standard reference for further information on the history of electrocardiography is: A History of Electrocardiography, G. E. Burch and N. P, DePascuale, Norman Publishing, San Francisco.
Since the implantation of the first cardiac pacemaker, implantable medical device technology has advanced with the development of sophisticated, programmable cardiac pacemakers, pacemaker-cardioverter-defibrillator arrhythmia control devices and drug administration devices designed to detect arrhythmias and apply appropriate therapies. The detection and discrimination between various arrhythmic episodes in order to trigger the delivery of an appropriate therapy is of considerable interest. Prescription for implantation and programming of the implanted device are based on the analysis of the PQRST electrocardiogram (ECG) that currently requires externally attached electrodes and the electrogram (EGM) that requires implanted pacing leads. The waveforms are usually separated for such analysis into the P-wave and R-wave in systems that are designed to detect the depolarization of the atrium and ventricle respectively. Such systems employ detection of the occurrence of the P-wave and R-wave, analysis of the rate, regularity, and onset of variations in the rate of recurrence of the P-wave and. R-wave, the morphology of the P-wave and R-wave and the direction of propagation of the depolarization represented by the P-wave and R-wave in the heart. The detection, analysis and storage of such EGM data within implanted medical devices are well known in the art. Acquisition and use of ECG tracing(s), on the other hand, has generally been limited to the use of an external ECG recording machine attached to the patient via surface electrodes of one sort or another.
The aforementioned ECG systems that utilize detection and analysis of the PQRST complex are all dependent upon the spatial orientation and number of electrodes available in or around the heart to pick up the depolarization wave front
As the functional sophistication and complexity of implantable medical device systems increased over the years, it has become increasingly more important for such systems to include a system for facilitating communication between one implanted device and another implanted device and/or an external device, for example, a programming console, monitoring system, or the like. For diagnostic purposes, it is desirable that the implanted device be able to communicate information regarding the device""s operational status and the patient""s condition to the physician or clinician. State of the art implantable devices are available which can even transmit a digitized electrical signal to display electrical cardiac activity (e.g., an ECG, EGM, or the like) for storage and/or analysis by an external device. The surface ECG, in fact, has remained the standard diagnostic tool since the very beginning of pacing and remains so today.
To diagnose and measure cardiac events, the cardiologist has several tools from which to choose. Such tools include twelve-lead electrocardiograms, exercise stress electrocardiograms, Holter monitoring, radioisotope imaging, coronary angiography, myocardial biopsy, and blood serum enzyme tests. Of these, the twelve-lead electrocardiogram (ECG) is generally the first procedure used to determine cardiac status prior to implanting a pacing system; thereafter, the physician will normally use an ECG available through the programmer to check the pacemaker""s efficacy after implantation. Such ECG tracings are placed into the patient""s records and used for comparison to more recent tracings. It must be noted, however, that whenever an ECG recording is required (whether through a direct connection to an ECG recording device or to a pacemaker programmer), external electrodes and leads must be used.
Unfortunately, surface electrodes have some serious drawbacks. For example, electrocardiogram analysis performed using existing external or body surface ECG systems can be limited by mechanical problems and poor signal quality. Electrodes attached externally to the body are a major source of signal quality problems and analysis errors because of susceptibility to interference such as muscle noise, power line interference, high frequency communication equipment interference, and baseline shift from respiration or motion. Signal degradation also occurs due to contact problems, ECG waveform artifacts, and patient discomfort. Externally attached electrodes are subject to motion artifacts from positional changes and the relative displacement between the skin and the electrodes. Furthermore, external electrodes require special skin preparation to ensure adequate electrical contact. Such preparation, along with positioning the electrode and attachment of the ECG lead to the electrode needlessly prolongs the pacemaker follow-up session. One possible approach is to equip the implanted pacemaker with the ability to detect cardiac signals and transform them into a tracing that is the same as or comparable to tracings obtainable via ECG leads attached to surface electrodes.
Previous art describes how to monitor electrical activity of the human heart for diagnostic and related medical purposes. U.S. Pat. No. 4,023,565 issued to Ohlsson describes circuitry for recording ECG signals from multiple lead inputs. Similarly, U.S. Pat. No. 4,263,919 issued to Levin, U.S. Pat. No. 4,170,227 issued to Feldman, et al, and U.S. Pat. No. 4,593,702 issued to Kepski, et al, describe multiple electrode systems which combine surface EKG signals for artifact rejection.
The primary use for multiple electrode systems in the prior art is vector cardiography from ECG signals taken from multiple chest and limb electrodes. This is a technique whereby the direction of depolarization of the heart is monitored, as well as the amplitude. U.S. Pat. No. 4,121,576 issued to Greensite discusses such a system.
Numerous body surface ECG monitoring electrode systems have been employed in the past in detecting the ECG and conducting vector cardiographic studies. For example, U.S. Pat. No. 4,082,086 to Page, et al., discloses a four electrode orthogonal array that may be applied to the patient""s skin both for convenience and to ensure the precise orientation of one electrode to the other. U.S. Pat. No. 3,983,867 to Case describes a vector cardiography system employing ECG electrodes disposed on the patient in normal locations and a hex axial reference system orthogonal display for displaying ECG signals of voltage versus time generated across sampled bipolar electrode pairs.
U.S. Pat. No. 4,310,000 to Lindemans and U.S. Pat. Nos. 4,729,376 and 4,674,508 to DeCote, incorporated herein by reference, disclose the use of a separate passive sensing reference electrode mounted on the pacemaker connector block or otherwise insulated from the pacemaker case in order to provide a sensing reference electrode which is not part of the stimulation reference electrode and thus does not have residual after-potentials at its surface following delivery of a stimulation pulse.
Moreover, in regard to subcutaneously implanted EGM electrodes, the aforementioned Lindemans U.S. Pat. No. 4,310,000 discloses one or more reference sensing electrode positioned on the surface of the pacemaker case as described above. U.S. Pat. No. 4,313,443 issued to Lund describes a subcutaneously implanted electrode or electrodes for use in monitoring the ECG.
Finally, U.S. Pat. No. 5,331,966 to Bennett, incorporated herein by reference, discloses a method and apparatus for providing an enhanced capability of detecting and gathering electrical cardiac signals via an array of relatively closely spaced subcutaneous electrodes (located on the body of an implanted device).
The present invention encompasses a leadless Subcutaneous Electrode Array (SEA) that provides various embodiments of a compliant surround shroud into which are placed electrodes embedded into recesses in the surround that is attached to the perimeter of the implanted pacemaker. These electrodes are electrically connected to the circuitry of the implanted pacemaker and detect cardiac depolarization waveforms displayable as electrocardiographic tracings on the pacemaker Programmer screen when the programming head is positioned above an implanted pacemaker (or other implanted device) so equipped with a leadless SEA.
The present invention provides a method and apparatus that may be implemented into the aforementioned medical devices to provide an enhanced capability of detecting and gathering electrical cardiac signals via an array of relatively closely spaced subcutaneous electrodes (located in a shroud placed circumferentially on the perimeter of an implanted device) which may be employed with suitable switching circuits, signal processors, and memory to process the electrical cardiac signals between any selected pair or pairs of the electrode array in order to provide a leadless, orientation-insensitive means for receiving the electrical signal from the heart.
The compliant surround shroud may consist of a non-conductive, bio-compatible urethane polymer, silicone, or softer urethane that retains its mechanical integrity during manufacturing and prolonged exposure to the hostile environment of the human body. The surround shroud placed around the perimeter of the pacemaker case of the subcutaneously implanted medical device has two preferred embodiments, the first with four and the second with three recesses, each of which contains an individual electrode. The four-electrode embodiment provides a better signal-to-noise ratio that the three-electrode embodiment. Embodiments, using a single electrode pair, are very sensitive to appropriate orientation of the device during and post implantation. Embodiments, using more than four electrodes increase complexity without any significant improvement in signal quality.
The preferred perimeter embodiment has electrodes connected to two amplifiers that are hardwired to two electrode pairs that record simultaneous ECGs. These ECGs are stored and sorted for processing in the pacemaker""s microprocessor. An alternative embodiment has electrodes on the face of the lead connector and/or face of the pacemaker that may be selectively or sequentially coupled in one or more pairs to the terminals of one or more sense amplifiers to pick up, amplify and process the electrical cardiac signals across each electrode pair. In one embodiment, the signals from the selected electrode pairs may be stored and compared to one another in order to determine the sensing vector that provides the largest cardiac signal (in a test mode). Following completion of the test mode, the system may employ the selected subcutaneous ECG signal vector for a number of applications.
The surround shroud and the electrode recesses are easy-to-mold. Since they also are better able to stand up to heat stress, they are easy to manufacture. The preferred perimeter embodiment (with four electrodes and two amplifiers) has several advantages. Since fewer amplifiers are needed, this embodiment saves power, thus extending battery life. The preferred embodiment also requires less memory, thereby allowing smaller electronic circuitry that can be manufactured less expensively.
Previous art found in U.S. Pat. No. 5,331,966 had electrodes placed on the face of the implanted pacemaker. When facing muscle, the electrodes were apt to detect myopotentials and were susceptible to baseline drift. The present invention minimizes myopotentials and allows the device to be implanted on either side of the chest by providing maximum electrode separation and minimal signal variation due to various pacemaker orientations within the pocket because the electrodes are placed on the surround shroud in such a way as to maximize the distance between electrode pairs.
The spacing of the electrodes in the present invention provides maximal electrode spacing and, at the same time, appropriate insulation from the pacemaker casing due to the insulative properties of the compliant shroud and the cup recesses into which the electrodes are placed. The electrode placement maintains a maximum and equal distance between the electrode pairs. Such spacing with the four-electrode embodiment maintains the maximum average signal due to the fact that the spacing of the two vectors is equal and the angle between these vectors is 90xc2x0, as is shown in mathematical modeling. Such orthogonal spacing of the electrode pairs also minimizes signal variation. An alternate three-electrode embodiment has the electrodes arranged within the surround shroud in an equilateral triangle along the perimeter of the implanted pacemaker. Vectors in this embodiment can be combined to provide adequate sensing of cardiac signals (ECGs).
Previous art (""966) had no recesses for the electrodes and, hence, made the device susceptible to motion detection. The present invention may recess the electrodes below the surface of the shroud, thereby eliminating interface with the body tissue, thus minimizing or eliminating motion artifact. An alternative embodiment, which provides an open cover for the electrodes and allows body fluids to come into contact with the electrode while minimizing the fluid movement, further enhances this effect. These covers also provide protection for the electrodes, so as to prevent damage during the implant procedure. The electrode covers also prevent contact with any other implanted devices which may be placed in the same pocket, as well as protection against any electro surgical electrodes used during the implant or explant procedures.
The electrodes"" surfaces require protection during handling as well as to prevent contamination. A coating, such as may be provided by Dexamethazone Sodium Phosphate, NaCL (salts) and sugar solutions, provides such protection as well as enhancing the wetting of the electrode surface after implant. Conductive hydro gels, applied wet and allowed to dry, may also be applied to the electrode surfaces to protect them from damage during handling and prevent contamination.
The present invention allows the physician or medical technician to perform leadless follow-up that, in turn, eliminates the time it takes to attach external leads to the patient. Such time savings can reduce the cost of follow-up, as well as making it possible for the physician or medical technician to see more patients during each day. Though not limited to these, other uses include: Holter monitoring with event storage, arrhythmia detection and monitoring, capture detection, ischemia detection and monitoring (S-T elevation and depression on the ECG), changes in QT interval, and transtelephonic monitoring.